The present disclosure relates to a draw-works. More specifically the disclosure relates to a draw-works comprising a first drum for storing and reeling an elongated hoisting member, a second drum for storing and reeling the elongated hoisting member and a rotation means for rotating the first and second drums. The disclosure also relates to a method for operating a draw-works.
A draw-works is the primary hoisting machinery that is a component of a drilling rig. Its main function is to provide a means of raising and lowering the travelling blocks, the travelling blocks further being connected to load suspension members. The wire-rope drilling line winds on the draw-work's drum and extends to the crown block and travelling blocks, allowing the drill string to be moved up and down as the drum turns. The segment of drilling line from the draw-works to the crown block is called the “fast line”. The drilling line then enters the sheaves of the crown block and makes several passes between the crown block and travelling block pulleys for mechanical advantage. The line then exits the last sheave on the crown block and is usually fastened to a derrick leg on the other side of the rig floor. This section of drilling line is called the “dead line”.
The term “draw-works” is often used interchangeably with “winch” to define the machinery in a hoisting system. Herein “draw-works” shall be taken to mean any hoisting machinery adapted to store and reel a wire rope or another elongated hoisting members, both as used on drilling rigs and in other hoisting operations such as with lifting cranes.
In the late 90's, draw-works including active heave compensation means became commercially available. The active heave compensated draw-works were AC-driven and featured single-speed, single-shaft, direct gear drive and a plurality of AC motors.
More recently, dual heave compensated draw-works have been introduced where one draw-works is placed on each side of the derrick, implying that the “dead line” is not fixed to the derrick or to the rig floor, but rather to another draw-works powered from an independent source. The dual draw-works offers the advantage of increased speed, as the wire rope may be reeled from both draw-works simultaneously, thus essentially doubling the speed of the travelling block. Moreover, the dual draw-works also offers the advantage of redundancy, in case one of the draw-works should fail, as well as simplified replacement of worn wire rope, so-called line exchange, as the wire rope may be reeled from one winch drum, over the pulley and to the second winch drum. Dual draw-works has the disadvantage of leaving a large footprint as a draw-works is placed on each side of the derrick, the available space often being limited on rig floors.
The requirements for hoisting capacity in terms of weight for draw-works, including for heave-compensated draw-works, are getting increasingly large, and the draw-works need to be dimensioned accordingly. Often it is the hoisting of casings or heavy blow-out preventers (BOPS) that sets the design load for the draw-works. At the same time, there is a strong desire to increase hoisting speed, particularly for increasing tripping speed, i.e. to reduce the time for pulling the drill string out of the hole or replacing it in the hole, which is the process in which a typical draw-works will be most frequently involved. This problem could be solved by doubling the motor capacity on one or both of the draw-works in a conventional dual draw-works system, but that would significantly increase the cost and weight of the system which already leaves a very large footprint.